From: Peter Zijlstra <peterz@infradead•org>
To: Michael Neuling <mikey@neuling•org>
Cc: "ak@linux•intel.com" <ak@linux•intel.com>,
LKML <linux-kernel@vger•kernel.org>,
Stephane Eranian <eranian@google•com>,
Linux PPC dev <linuxppc-dev@ozlabs•org>,
Ingo Molnar <mingo@kernel•org>
Subject: Re: [PATCH 3/3] perf, x86, lbr: Demand proper privileges for PERF_SAMPLE_BRANCH_KERNEL
Date: Fri, 17 May 2013 13:39:22 +0200 [thread overview]
Message-ID: <20130517113922.GF5162@dyad.programming.kicks-ass.net> (raw)
In-Reply-To: <29969.1368790328@ale.ozlabs.ibm.com>
On Fri, May 17, 2013 at 09:32:08PM +1000, Michael Neuling wrote:
> Peter Zijlstra <peterz@infradead•org> wrote:
> > Wouldn't it be mostly conditional branches that are the primary control flow
> > and can get predicted wrong? I mean, I'm sure someone will miss-predict an
> > unconditional branch but its not like we care about people with such
> > afflictions do we?
>
> You could mispredict the target address of a computed goto. You'd know
> it was taken but not know target address until later in the pipeline.
Oh right, computed targets could indeed be mis predicted. I was more thinking
about jumps with immediate values.
> On this, the POWER8 branch history buffer tells us two things about the
> prediction status.
> 1) if the branch was predicted taken/not taken correctly
> 2) if the target address was predicted correctly or not (for computed
> gotos only)
> So we'd actually like more prediction bits too :-D
So if I understand this right, 1) maps to the predicted flags we have; 2)
would be new stuff?
We don't really have anything like that on x86, but I suppose if you make the
thing optional and present a 'useful' use-case implemented in userspace code
we could take it :-)
> > Anyway, since PPC people thought it worth baking into hardware,
> > presumably they have a compelling use case. Mikey could you see if you
> > can retrieve that from someone in the know? It might be interesting.
>
> I don't think we can mispredict a non-conditional non-computed but I'll
> have to check with the HW folks.
I was mostly wondering about the use-case for the conditional filter. Stephane
didn't think it useful, clearly your hardware guys thought different :-)
next prev parent reply other threads:[~2013-05-17 11:41 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20130503121122.931661809@chello.nl>
[not found] ` <20130503121256.230745028@chello.nl>
[not found] ` <CABPqkBTvcQAyXfeiJwpPsNtbs4ffsVguXE631dK_rbzHpwBdOg@mail.gmail.com>
[not found] ` <20130516090916.GF19669@dyad.programming.kicks-ass.net>
2013-05-16 10:09 ` [PATCH 3/3] perf, x86, lbr: Demand proper privileges for PERF_SAMPLE_BRANCH_KERNEL Michael Neuling
2013-05-16 10:15 ` Michael Neuling
2013-05-16 11:16 ` Peter Zijlstra
2013-05-16 15:36 ` Stephane Eranian
2013-05-17 11:12 ` Peter Zijlstra
2013-05-17 11:32 ` Michael Neuling
2013-05-17 11:39 ` Peter Zijlstra [this message]
2013-05-17 21:39 ` Stephane Eranian
2013-05-17 22:14 ` Michael Neuling
2013-05-17 22:59 ` Stephane Eranian
2013-05-21 5:41 ` Michael Neuling
2013-05-21 8:50 ` Peter Zijlstra
2013-05-21 13:46 ` Stephane Eranian
2013-05-21 13:55 ` Stephane Eranian
2013-05-22 6:43 ` Anshuman Khandual
2013-05-22 12:23 ` Stephane Eranian
2013-05-22 14:51 ` Anshuman Khandual
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